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What Are The Reasons For Tensile Cracking Of Stainless Steel
Original: | Author: DGMF Mold Clamps | Publish Time: 2020-08-03 | 7983 Views | Share:
What are the reasons for the cracking of stainless steel material during stretching?

What Are The Reasons For Tensile Cracking Of Stainless Steel? - DGMF Mold Clamps Co., Ltd

What are the reasons for the cracking of stainless steel material during stretching?

Stainless steel has a low elongation rate, a large elastic modulus E, and a high hardening index.

The tensile cracking of the stainless steel plate sometimes occurs after the tensile deformation, sometimes immediately when the tensile part is withdrawn from the die; sometimes it occurs when the tensile deformation is subjected to impact or vibration; sometimes after the tensile deformation It happens after a period of storage or during use.

Analysis of common problems in stainless steel stretching process:

1. Causes of cracking

Austenitic stainless steel has a high cold work hardening index (stainless steel is 0.34). Austenitic stainless steel is meta-stable and undergoes phase transformation during deformation, which induces martensite phase.

Martensite is relatively brittle, so it is prone to cracking. During plastic deformation, as the amount of deformation increases, the content of induced martensite will also increase with the increase of the amount of deformation, and the greater the residual stress. The relationship between residual stress and martensite content: induced horse The higher the content of the tensite phase, the greater the residual stress caused, and the easier it is to crack during processing.

2. Causes of surface scratches

The scratches on the surface of stainless steel drawing parts are mainly due to the relative movement of the workpiece and the mold surface. Under a certain pressure, the blank and the local surface of the mold are directly rubbed, and the deformation heat of the blank causes the blank and metal chips to be deposited on the mold. On the surface, scratches on the surface of the workpiece.

Preventive measures for common forming defects of stainless steel:

1. Choose the right stainless steel material: the commonly used materials in austenitic stainless steel are 1Cr18Ni9Ti and 0Cr18Ni9Ti. In the deep drawing process, 1Cr18Ni9Ti is more stable than 0Cr18Ni9Ti and has better crack resistance. Therefore, 1Cr18Ni9Ti should be selected as much as possible.

2. Reasonable selection of mold material. Stainless steel hardens significantly during deep drawing, producing many hard metal points, causing adhesion, and making the workpiece and mold surface easy to scratch and wear. Therefore, general mold tool steel cannot be used. 

The practice has proved that the choice of copper-based alloy molds can eliminate scratches and scratches on the surface of stainless steel parts and reduce the damage rate. The other material is high-aluminum copper-based alloy mold material (containing aluminum 13Wt%

This material has low miscibility with SUS304 stainless steel, and there is no adhesion between the drawn part and the mold, and the surface of the drawn part is not easy to scratch Scratch, low product polishing cost, has been successfully applied in the field of stainless steel deep drawing.

However, due to the low hardness of this mold (40HRC
45HRC), it is often used to produce products with a relatively small thickness t/D. Generally, after 1500 to 2000 pieces are drawn, it is easy to produce radially drawn edges on the surface of the concave die starting from the round corner.

Silicon nitride ceramics (Si3N4) has become an important engineering material, especially reaction sintered silicon nitride ceramics, which have good high and low-temperature mechanical properties, thermal shock resistance and chemical stability, and can be easily made into complex shapes Components. The high hardness, high wear resistance, and high chemical stability of ceramic materials can be used to draw SUS304 stainless steel with reaction sintered silicon nitride molds instead of metal molds.

3. Choose reasonable convex and concave die fillets. The die fillet has a great relationship with stress size and distribution. The radius of the fillet is large, and the pressing area of the blank holder is insufficient, which is prone to instability and wrinkles; if the fillet is too small, the resistance of the material into the die during the deformation process will increase, and the material will not easily flow and transfer inward. Increase the maximum tensile stress in the force transmission area, which may cause tensile cracks. 

Therefore, it is very important to choose a reasonable radius of convex and concave corners. When the relative fillet radius RP/t of the punch is about 4, it is most beneficial to prevent cracking. The relative fillet radius of the concave mold and the convex mold increases, and the extreme deformation degree will increase. When the relative fillet radius of the concave mold is 5mm to 8mm, it is helpful to prevent cracking.

4. The researchers who used the tape thinning for deep drawing have also proved through experiments that the use of tape thinning for deep drawing can greatly reduce the maximum value of the tangential residual stress of the drawn part and effectively prevent the occurrence of longitudinal cracks. According to the different degrees of deformation and original sheet thickness, choose the appropriate thinning factor Ψn (generally 0.9t
0.95t). If the value of Ψn is too small, the deformation stress will increase sharply, which will cause the bottom of the drawing part to crack.

5. Adding an intermediate annealing process to the deep drawing process can completely eliminate residual stress and restore the structure of austenitic stainless steel. For high hardness stainless steel, intermediate annealing is required. Such as 1Crl8Ni9Ti, usually the heating temperature is 1150
1170, the heating time is 30min, and it is cooled in airflow or water.

In addition, whether it is heat treat
ment between processes or the final product, it should be carried out as soon as possible after drawing to avoid deformation or cracking of the workpiece due to internal stress due to long-term storage.

6. Use appropriate lubricants. The use of appropriate lubricants has a significant effect on the deep drawing of stainless steel. The lubricant can form a film with a certain degree of toughness and elongation between the convex and concave dies, which is beneficial to the deep drawing of stainless steel. 

For stainless steel deep-drawn parts with large drawing deformation and difficult forming, polyvinyl fluoride film can be used as a lubricant in actual production. Polyvinyl fluoride film has excellent tear strength, certain toughness, and elongation, and is easy to clean. After coating the dry film, the dry film can deform with the blank during the deep drawing process, and the blank can be always separated from the mold. 

In addition, the film itself has a certain porosity and a large number of fiber cracks, so a certain amount of lubricant can be stored, so the film Equivalent to a layer of dry film lubricant. This lubrication method can effectively isolate the deformed stainless steel plate from the surface of the mold and has a good lubrication effect, which is beneficial to improve the service life of the mold and the pass rate of the product.

7. Other preventive measures

The following precautions can also be taken in the deep drawing of stainless steel: 

(1) Because white cast iron has good oil storage performance, it is easy to form a lubricating oil film, the material of the blank holder can be white cast iron; 

(2) The tapered blank holder Circle; 

(3) The edge processing is smooth, and there should be no micro cracks.